Noise and channel estimation using low spreading factors

ABSTRACT

Noise measurements are made within a fraction of a single symbol period of a longest orthogonal code symbol. A control processor identifies an unoccupied code having a spreading factor that is less than a longest spreading factor for the system. A despreader measures symbol energy based on the unoccupied code and a noise estimator generates noise estimations based on the measured symbol energies. The subscriber station uses similar techniques in order to perform channel estimations within a period that is a fraction of a symbol period of a longest-spreading-factor code.

CROSS-REFERENCES

The present application is a continuation of U.S. application Ser. No.10/290,749, filed Nov. 7, 2002, now U.S. Pat. No. 7,254,170, whichapplication claims the benefit of U.S. provisional application Ser. No.60/424,474, filed Nov. 6, 2002. Application Ser. No. 10/290,749 ishereby incorporated by reference.

BACKGROUND

1. Field

The present disclosure relates generally to wireless communication, andmore specifically to improved noise estimation in a wirelesscommunication system.

2. Background

The field of wireless communication includes many wireless applicationssuch as voice communication, paging, packet data services, andvoice-over-IP. One challenge presented by such services are the widelyvarying requirements for capacity, quality-of-service, latency, datarates in the different services. Various over-the-air interfaces havebeen developed to accommodate combinations such services using differentwireless communication techniques such as frequency division multipleaccess (FDMA), time division multiple access (TDMA), and code divisionmultiple access (CDMA).

In order to accommodate combinations services having different sets ofrequirements, communication standards such as the proposed cdma2000 andW-CDMA specify the use of orthogonal codes of varying length on thedownlink channels from a wireless base station to a subscriber station.Some standards also specify transmitting signals for different uplinkchannel (in the direction from the subscriber station to the basestation) using orthogonal codes of varying lengths for the differentchannels. For example, a wireless base station may transmit three typesof downlink signals, pilot, voice, and packet data, using a differentlength orthogonal code symbols to channelize or “cover” each differenttype of signal. The length of an orthogonal code symbol is typicallydescribed as a number of “chips,” with a chip being a smallest binarycomponent of a transmitted signal. In a spread spectrum system, each bitof information is multiplied by a sequence of binary chips having apredetermined number of chips-per-bit. Multiplying a single binaryinformation bit by an orthogonal code symbol effectively “spreads” theinformation bit over all of the chips in the symbol. For this reason,the chip length or chips-per-bit is often referred to as a “spreadingfactor” of a transmitted information signal.

Another aspect of current spread spectrum systems is the sharing offrequency bands between different subscriber stations and betweendifferent base stations. In other words, neighboring base stations in aspread spectrum system transmit their downlink signals in the samefrequency band as each other. Because of this sharing (also called“reuse”) of frequency bands, downlink signals transmitted by a basestation destructively interfere with the downlink signals of neighboringbase stations. This interference decreases the capacity of theneighboring base stations, measured in either number of subscriberstations that can be supported or maximum information throughputpossible on the downlink. The capacity of such systems can be increasedby using power control techniques to decrease the transmit power of allsignals to a lowest value that will still permit the signals to becorrectly received and decoded by subscriber stations. The effectivenessof power control depends largely on the accuracy of noise measurementsmade by subscriber stations and base stations. There is therefore a needin the art for a way to provide maximal noise measurement accuracy insystems utilizing varying spreading factors.

SUMMARY

Embodiments disclosed herein address the above-stated needs by enablingnoise measurements to be made within a fraction of a single symbolperiod of a longest orthogonal code symbol. In a first aspect, asubscriber station in a wireless system determines when information isbeing transmitted on each of the first two channels spread using alongest spreading factor for the wireless system, determines also thatthe following two channels spread using the longest spreading factor areunoccupied (not being used to transmit data), and performs noiseestimation over a period that is a fraction of a symbol period of alongest-spreading-factor code symbol. In a second aspect, a subscriberstation determines an unused channel associated with ashort-spreading-factor code symbol and performs noise measurement withinthe symbol period of the short-spreading-factor code. In a third aspect,the subscriber station uses similar techniques in order to performchannel estimations within a period that is a fraction of a symbolperiod of a longest-spreading-factor code.

The subscriber station may independently identify of the unused channel.Alternatively, a base station may provide the identity of the unusedchannel to the subscriber station.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements.

FIG. 1 shows an illustrative wireless communication system;

FIG. 2 shows an illustrative coding tree having an unusedshort-spreading-factor channel;

FIG. 3 shows an illustrative method for measuring noise based on usageof a first set of longest-spreading-factor codes sharing a common parentcode;

FIG. 4 shows an illustrative method for measuring noise based on anunoccupied short-spreading-factor code;

FIG. 5 shows an illustrative subscriber station for measuring noiseusing short-spreading-factor codes; and

FIG. 6 shows an illustrative base station apparatus for providingtransmission signals that allow noise estimation using short-SF codes.

DETAILED DESCRIPTION

FIG. 1 shows an illustrative wireless communication system with a singlebase station including a subscriber station 102 with an antenna 104communicating with a base station 112 having an antenna 114. Basestation 112 or subscriber station 102 may use multiple antennas withoutdeparting from the embodiments described herein. Base station 112transmits wireless signals to subscriber station 102 on downlink channel124. Subscriber station 102 transmits wireless signals to base station112 on uplink channel 122. Base station 112 transmits multiple signalson downlink channel 124 using different sub-channels. In an illustrativeembodiment, the different downlink sub-channels are distinguished usingorthogonal codes. In other words, each bit of data (data bit) to betransmitted on a particular sub-channel is spread by multiplying thedata bit by an orthogonal code symbol that is unique among the codesused by the base station 112. In an illustrative embodiment, subscriberstation 102 utilizes knowledge of the orthogonal code symbols used toencode the different sub-channels, along with knowledge of whichsub-channels are being used to carry data, to perform measurements ofthe noise received on downlink 124.

As used herein, the term “noise” includes interference from thermalnoise as well as interference caused by signals transmitted from othersources. As used herein, a subscriber station may be mobile orstationary, and may communicate with one or more base stations. Asubscriber station may further be any of a number of types of devicesincluding but not limited to PC card, compact flash, external orinternal modem, or wireless phone, and may include a wireline phone ormodem.

The techniques that make possible improvements in noise measurements arefacilitated by referring to FIG. 2, showing an illustrative coding treeshowing assignments of orthogonal code symbols to differentsub-channels. The illustrative embodiment shown in FIG. 2 is based onsub-channel designations described in the W-CDMA standard but areapplicable to any wireless signal utilizing spreading codes of varyinglength for different sub-channels. The coding tree 220 in FIG. 2 shows amethodical approach to allocating orthogonal code symbols of varyingchip lengths to different sub-channels. Where each data bit ismultiplied by an entire code symbol, the length of the code symbol inchips is equal to the spreading factor for the sub-channel.

In FIG. 2, each “parent” orthogonal code of a first symbol length can beused to derive two “child” orthogonal codes having twice that symbollength. The levels of orthogonal code are represented using the notationC(X,Y), where X is the number of chips in the orthogonal code, and Y isa sequence number of the codes at that level from top to bottom. Thevalue X may also be viewed as the spreading factor (SF) of thecorresponding code. When two child orthogonal codes are derived from aparent orthogonal code, the first of the two codes will be the parentcode repeated twice, and the second of the two codes will be the parentcode followed by the negative of that code. Thus, the two-chip C(2,0)code having the chip sequence (+,+) splits into two four-chip childcodes, C(4,0)=(+,+,+,+) and C(4,1)=(+,+,−,−). Similarly, the two-chipC(2,1) code having the chip sequence (+,−) splits into two four-chipchild codes, C(4,2)=(+,−,+,−) and C(4,3)=(+,−,−,+). Constructed thisway, all the codes at the “leaf” nodes of the tree are guaranteed to beorthogonal to all other leaf-node codes, even if the codes at some leafnodes are longer (contain more chips) than other leaf-node codes.

Although code tree 220 is shown with the longest “branches” at the top,i.e. such that the longest codes have lower values for Y in the C(X,Y)notation, nothing precludes use of a code tree that has long branchesextending to the right from lower branches. For example, the C(4,0) andC(4,3) branches of the tree may extend all the way to leaf-node codes oflength 256 chips at the same time that C(4,1) and C(4,2) are used asleaf node codes.

As mentioned above, the illustrative channel allocation shown in FIG. 2is based on sub-channel designations described in the W-CDMA standard.Accordingly, the C(256,0) code at code tree branch 202 is used to spreada pilot sub-channel known as the Common Pilot Channel (CPICH) and theC(256,1) code at code tree branch 204 is used to spread a Primary CommonControl Physical Channel (P-CCPCH). The longest code in code tree 220 is256 chips. Thus, 256 chips is called the longest spreading factor ofcode tree 220, and any code in code tree 220 having a length of 256chips is called a longest-spreading-factor code. The CPICH and P-CCPCHare longest-spreading-factor codes in the W-CDMA code tree.

As mentioned above, contemplated spread spectrum systems utilizedifferent spreading factors for different channels based on therequirements of the service associated with the channel. For example, inW-CDMA, a pilot signal is covered using pilot channel symbols having alength of 256 chips, a voice signal is covered using channel symbolshaving a length of 128 chips, and a packet data signal may be coveredusing channel symbols having a length of 64 chips. Thus, the spreadingfactors of the pilot, voice, and packet data channels are 256, 128, and64, respectively. Additionally, some low-vocoder-rate voice signals canalso be covered using 256-chip channel symbols.

Where it is not known which channels are being used to carry informationsignals, noise measurements may only be made over the longest spreadingfactor. In other words, in a W-CDMA system using the code tree shown inFIG. 2, if a subscriber station does not know which channels remainunallocated, the subscriber station may only generate noise measurementsover the longest spreading factor of 256 chips.

Improved noise measurement is possible, however, where the subscriberstation knows that one or more channels having a shorter spreadingfactor. In an illustrative embodiment, the subscriber station determinesthat a channel having a spreading factor of 128 chips is unused andperforms noise measurements over 128-chip periods. The result is thattwo 128-chip noise measurements can be obtained for each 256-chipperiod. For example, if the subscriber station determines that theC(128,1) code at code tree branch 206 is not being used to transmit asignal from a base station, the subscriber station can generate two128-chip noise measurements for each 256-chip period spanned by theCPICH C(256,0) code at code tree branch 202 or by the C(256,1) P-CCPCHcode at code tree branch 204.

Where no unused channel can be identified, noise estimates cannot begenerated more frequently than once every 256 chips. Such noiseestimates are less preferred because they require a longer time togenerate and are less accurate than noise estimates made over a shorterperiod and then averaged together. Improved noise estimates can begenerated where select codes in the code tree 220 are known by thesubscriber station to be unoccupied (not being used to transmit data).

In order for a code to be considered unoccupied, all child codes of thecode must also be unoccupied. For example, in order for the 32-bitC(32,31) code 218 to be considered unoccupied, both 64-bit child codesC(64,62) 210 and C(64,63) 216 must be unoccupied. In order for the64-bit C(64,63) 216 code to be considered unoccupied, both 128-bit childcodes C(128,126) 212 and C(128,127) 214 must be unoccupied. A code canbe considered unoccupied when its corresponding code channel is nottransmitted at all or is transmitted at zero power.

FIG. 3 shows an illustrative method for performing improved noiseestimates using knowledge of unoccupied code channels. The subscriberstation determines the usage of a first four longest-spreading-factorcodes at 304. The ways in which the subscriber station can make thisdetermination include receiving a downlink message indicating the usageof these codes at 302. Alternatively, the subscriber station may performmeasurements of received signals to independently determine which of thechannels associated with the longest-spreading-factor codes are beingused to transmit signals.

To enable improved noise estimation, the four longest-spreading-factorcodes must share a common parent code. For example, the subscriberstation may know a priori that the first longest-spreading-factor code,C(256,0) 202, is used to transmit an all-ones pilot channel (CPICH) andthat the second longest-spreading-factor code, C(256,1) 204, is used totransmit P-CCPCH data. The subscriber station may receive a downlinkmessage at 302 that indicates that both of the remaining codes under theparent code C(64,0) 208 are unoccupied. Specifically, the downlinkmessage received at 302 is indicative of whether both the C(256,2) andC(256,3) codes are unoccupied or whether the parent C(128,1) code 206 isunoccupied. In an illustrative embodiment, the downlink message receivedat 302 is a single bit. The downlink message can be received from a basestation on a dedicated channel, a broadcast channel, or a multicastchannel.

When the subscriber station determines that half of the four child codesof a single parent code are unoccupied, the subscriber station cangenerate two noise estimates for each two code symbol periods of theparent code. Specifically, the subscriber station despreads the receivedsignal using the parent code at 306, to provide a short-SF-despreadsignal. The short-SF-despread signal is then buffered during a firstparent code symbol period at 308. During a second parent code symbolperiod, the short-SF-despread signal is buffered again at 310. Adifference between the values buffered during the first parent codesymbol period and the values buffered during the second parent codesymbol period is determined at 312. The difference established at 312constitutes a noise estimate for the receive channel. This differencecan be determined by subtracting the first set of values from thesecond.

It is important to point out that the signals corresponding to parentcode symbol periods that may be subtracted from each other mustcorrespond to parent code symbol periods in which it is known that thedata transmitted on the two occupied channels is identical. For example,when the subscriber station determines that both C(256,2) and C(256,3)are unoccupied, the subscriber station then knows that the chip valuestransmitted in the first 64-chip period are identical to the chip valuestransmitted in the second 64-chip period. Specifically, the first 64chips of the CPICH code are identical to the second 64 chips of theCPICH code, and the first 64 chips of the P-CCPCH code are identical tothe second 64 chips of the P-CCPCH code. Accordingly, any differencebetween the received chip energy values from the first 64-chip periodand the second 64-chip period can only be attributed to noise.Similarly, the difference between the received chip energy values fromthe third 64-chip period and the fourth 64-chip period can only beattributed to noise. Therefore, when the subscriber station knows thatthe C(256,2) and C(256,3) codes are unoccupied, the subscriber stationcan generate one noise estimate during each of the first two 64-chipperiods and the last two 64-chip periods of every of every 256-chip codesymbol duration. These two noise estimates, requiring only 128 chipseach to generate, may be used directly or averaged together to form areliable noise estimate for the receive channel.

In an illustrative embodiment, the difference between receivedinterference energy values are determined on a 64-chip symbol-by-symbolbasis, for example, using a fast Hadamard transform (FHT). In anillustrative embodiment, each 256-chip code symbol period is dividedinto two successive 128-chip code symbol periods. Each of those two128-chip code symbol periods is then divided into two successive 64-chipcode symbol periods. The difference between the symbol energy over thefirst 64-chip period and the second 64-chip period are measured toprovide a first noise estimate over the first 128-chip code symbolperiod. The difference between the symbol energy over the third 64-chipperiod and the fourth 64-chip period are measured to provide a firstnoise estimate over the second 128-chip code symbol period. Where, as inW-CDMA, the 64-chip parent code of the CPICH and P-CCPCH channels is theC(64,0) all-ones code 208, the 64-chip symbol energy measurements can begenerated without actually multiplying the received samples by theC(64,0) code 208. Such multiplication is unnecessary, becausemultiplying the received samples by the all-ones code leaves thereceived samples unchanged.

The knowledge that the C(128,1) code 206 can also be used by thesubscriber station to perform improved channel estimates for thewireless channel. Specifically, in W-CDMA the CPICH channel is a pilotchannel that is never used to carry data. The CPICH channel is providedas a phase and amplitude reference for performing channel estimates thatenable coherent demodulation of downlink signals. Where the C(128,1)code 206 is used to transmit data, channel estimates using the CPICHcode 202 can only be generated once per 256-chip CPICH code symbolperiod. However, where the subscriber station knows that the P-CCPCHchannel is transmitted at a 90% duty-cycle as described above, thenduring the periods in which P-CCPCH code is unoccupied, a channelestimate can be generated every 128-chip code symbol period using theparent C(128,0) code as the pilot code. The subscriber station cangenerate two channel estimates within a single 256-chip code symbolperiod.

In addition, where the subscriber station also knows that the C(128,1)code 206 is unoccupied, then during the periods in which P-CCPCH code isunoccupied, channel estimates can be generated every 64 chip code symbolperiod. Specifically, during the 10% of the time that the CPICH code isthe only child code of parent code C(64,0) 208 that is occupied, thenthe parent code C(64,0) 208 can be used as a 64-chip pilot code. Thesubscriber station generates one channel estimate per 64-chip codesymbol period, or four channel estimates per 256-chip code symbolperiod. In an illustrative embodiment, the subscriber station usesinfinite-impulse-response (IIR) or finite-impulse-response (FIR)filtering of these 64-chip channel estimates to generate the channelestimates that are actually used for coherent demodulation of theremaining channels. Alternatively, the subscriber station may use thechannel estimates directly without filtering.

FIG. 4 shows another way to measure noise when the subscriber hasknowledge of an unused code having a spreading factor shorter than thelongest-spreading-factor code. The subscriber station identifies such anunused code at 404. The ways in which the subscriber station canidentify the unused code include receiving a downlink messageidentifying the unused code at 402. Alternatively, the subscriberstation may perform measurements of received signals to independentlyidentify an unused code.

Once the subscriber station has identified an unused code, thesubscriber station can generate a noise estimate for every symbol periodcorresponding to the unused short-SF code. The subscriber stationdespreads the received signal using the unused code at 406, to provide ashort-SF-despread signal. The subscriber station then generates a noiseestimate from the short-SF-despread signal at 408.

For example, if the unused code has a spreading factor of 64 such thatthe code symbol length of the unused code has a length of 64 chips, thesubscriber station can generate at 408 a new noise estimate every 64chips. Alternatively, the subscriber station could generate fourseparate noise estimates for every 256 chips. The four noise estimatescould be averaged together or used separately. Alternatively, the noiseestimates could be entered into an infinite impulse response (IIR)filter or a finite impulse response (FIR) filter. Averaging the four64-chip estimates for every 256-chip period may be viewed as a specificembodiment of an FIR filter.

Because the subscriber station knows that no signal is transmitted usingthe identified unused code, and also because the subscriber stationknows that all other codes are orthogonal to the unused code, anydifference between the received chip energy values received from oneunused code symbol period and the next can only be attributed to noise.In an illustrative embodiment using a 64-chip unoccupied code, thesubscriber station determines the signal energy for each 64-chip periodand then determine a difference between that symbol energy value and thesymbol energy value from the previous 64-chip period. In an illustrativeembodiment, the symbol energy values are determined using a fastHadamard transform (FHT). The short-SF noise estimates may be utilizeddirectly by the subscriber station, or may first be filtered using aninfinite impulse response (IIR) or finite impulse response (FIR) filter.On example of an FIR filter is averaging four consecutive noisemeasurements to generate a 256-chip noise estimate for each 256-chipcode symbol period.

In proposed W-CDMA systems, the CPICH channel is not used to carry datainformation. Specifically, the CPICH channel is a pilot channel thatremains at a constant value. Where the CPICH channel is the all-onesC(256,0) code 202, the CPICH channel represents a constant DC signalthat is effectively unmodulated by any orthogonal code. Another aspectof proposed W-CDMA systems is the discontinuous transmission of theP-CCPCH channel. Specifically, the P-CCPCH channel is transmitted at a90% duty-cycle, such that the corresponding P-CCPCH code 204 isunoccupied during every tenth 256-chip code symbol period. Thesubscriber station can easily determine the phase of these cycles andcan therefore identify the one out of ten 256-chip code symbol periodsduring which the P-CCPCH code 204 is unoccupied. Where the remaining128-chip code 206 is also known to be unoccupied, this allows thesubscriber station to treat the 64-chip parent code C(64,0) 208 asessentially unoccupied. Accordingly, in an illustrative embodiment thesubscriber station may alternate between the noise estimation techniquesshown in FIG. 3 and FIG. 4 based on when the P-CCPCH code 204 isoccupied. Specifically, during nine out of ten 256-chip code symbolperiods, the subscriber station performs noise estimation as describedin conjunction with FIG. 3. However, during every tenth 256-chip codesymbol period the subscriber station generates one noise estimate foreach of the three transitions between 64-chip segments of the 256-chipcode symbol period.

During every tenth 256-chip code symbol period, when the P-CCPCH channelis not transmitted, the 256-chip code symbol period is divided into foursequential 64-chip periods, the subscriber station measures theinterference energy for each of the four 64-chip periods at 406, andgenerates noise estimates at 408 by determining the difference betweenthe interference energy measurements. A first noise estimate isgenerated from the symbol energy difference between the first and second64-chip periods, a second noise estimate is generated from the symbolenergy difference between the second and third 64-chip periods, and athird noise estimate is generated from the symbol energy differencebetween the third and fourth 64-chip periods. In an alternateembodiment, the subscriber station can generate more than three noisemeasurements in the tenth 256-chip code symbol period by determining thedifference between non-adjacent 64-chip periods. For example, a fourthnoise estimate could be generated by determining a difference betweeninterference measurements during the first and fourth 64-chip segmentswithin the 256-chip code symbol period.

Alternatively, where the subscriber station knows of an additionalunoccupied code that is not in the C(64,0) code tree branch, thesubscriber station may perform simultaneous noise estimations usingmultiple unoccupied codes and combine the noise estimates. For example,where both the C(128,1) code 206 and the C(64,62) code 210 are bothknown to be unoccupied, then every tenth 256-chip code symbol periodwhen the P-CCPCH 204 code is unoccupied, the subscriber station couldcombine the noise estimates generated using the C(64,0) 208 and C(64,62)210 code symbols. Additionally, noise estimates using code symbols ofdifferent chip lengths could also be combined to create more accuratecombined noise estimates. For example, where the subscriber stationknows that codes C(64,62) 210 and C(128,126) 212 are both unoccupied,the subscriber station may generate noise estimates using bothunoccupied codes and then combine the noise estimates to form onecombined noise estimate for every 128 or every 256 chips.

FIG. 5 shows an illustrative apparatus for subscriber station 102,configured to measure noise based on usage of a first set oflongest-spreading-factor codes sharing a common parent code, asdiscussed in relation to FIG. 3. A wireless signal is received throughantenna 104. As mentioned above, subscriber station 102 may use multipleantennas without departing from the embodiments described herein. Thegain of the signal is adjusted in automatic gain control (AGC) 504before being sampled at sampler 506. AGC 504 may also include either adigital or analog downconverter to downconvert the received signal to anintermediate or baseband frequency. The sampled signal is then providedto descrambler 508, where the samples are multiplied by a scramblingcode generated by scrambling code generator 518 to provide a descrambledsignal.

Sampler 506 may perform real sampling, generating a single stream ofdigital samples. Alternatively, sampler 506 may perform complex samplingproviding a complex sample stream having an in-phase signal componentand a quadrature-phase signal component. Sampler 506 may sample thereceived signal at the rate of one sample per chip (commonly referred toas “chip-x-1”) or may sample at a higher rate such as four samples perchip (commonly referred to as “chip-x-4”).

Scrambling code generator 518 may generate a real scrambling coderepresented by a single stream of digital values or may generate acomplex scrambling code having a real component and an imaginarycomponent. Where either the scrambling code or the sample stream areexclusively real, descrambler 508 performs its descrambling functionusing real multiplication. Where both the scrambling code and the samplestream are complex, descrambler 508 performs complex multiplication ofthe complex sample stream by the complex scrambling code. Such complexmultiplication is described in detail in U.S. Pat. No. 5,930,230 [the'230 patent]. The subscriber station 102 may also utilize a pilot filter(not shown) in order to correct for phase errors inherent in thechannel. A detailed example of a real pilot filter is provided in U.S.Pat. No. 5,506,865, and an example of a complex pilot filter is providedin the aforementioned '230 patent.

In an illustrative embodiment, the scrambling code generated byscrambling code generator 518 corresponds to a scrambling code used byan individual base station. In an illustrative embodiment, each basestation uses a different scrambling code to scramble all itstransmissions in order to provide orthogonality or near orthogonalitybetween the signals of one base station and the next. For example, somebase stations scramble their transmissions using a common pseudonoise(PN) sequence that is offset in time to provide near orthogonality. Thebase stations may use the PN sequence until it repeats (for example,every 26.7 milliseconds) or may truncate and restart the PN sequence atregular intervals (for example, every 10 milliseconds at the beginningof a message frame). The use of other types of scrambling codes is alsoanticipated.

The descrambled signal provided by descrambler 508 is multiplied by ashort-spreading-factor (short-SF) code in short-SF despreader 512. Theshort-SF code used by short-SF despreader 512 is determined from acontrol signal from control processor 520. For example, where thesubscriber station is receiving both a CPICH and a P-CCPCH signal, butwhere the next 128-chip code 206 is known to be unoccupied, short-SFdespreader 512 despreads the received samples in 64-chip incrementsusing the C(64,0) code 208. One of skill in the art will recognize thatdespreading using an all-ones code such as C(64,0) does not require anyactual multiplication by a code, but can be accomplished by merelybuffering the chip energy values. In an illustrative embodiment,short-SF despreader 512 buffers the first 64 chip energy values of each128-chip code symbol period on a chip-by-chip basis in a buffer (notshown). During the second 64-chip period of each 128-chip code symbolperiod, short-SF despreader 512 determines the difference between thechip energy in each of the 64 chip periods and the corresponding chipenergy from the previous 64-chip period and provides each of the 64 chipenergy difference values to noise estimator 516. Noise estimator 516uses these chip energy difference values to determine a noise estimatefor each 128-chip code symbol period. Alternatively, short-SF despreader512 provides a single interference measurement for each whole 64-chipcode symbol period, and noise estimator 516 uses a pair of interferencemeasurements to generate a noise estimate for each 128-chip code symbolperiod. Noise estimator 516 provides the noise estimations to controlprocessor 520. Thus, in an illustrative embodiment, the subscriberstation apparatus 102 operates in accordance with the method describedin FIG. 3.

Alternatively, the subscriber station apparatus 102 can also be operatedin accordance with the method described in FIG. 4. For example, wherethe subscriber station determines that a short-SF code outside the codetree branch C(64,0) 208 is unoccupied, the subscriber station cangenerate a new noise estimate after each code symbol periodcorresponding to the unoccupied code. In that case, control processor520 provides a signal to short-SF despreader 512 identifying theunoccupied short-SF code to be used for noise measurements. In anillustrative embodiment, short-SF despreader 512 utilizes a buffer (notshown) having the same length as the unoccupied short-SF code andprovides to noise estimator 516 chip-by-chip energy differences from oneunoccupied-code-symbol period to the next. Alternatively, short-SFdespreader 512 can generate a single interference measurement value foreach code symbol period corresponding to the unoccupied code symbol. Inan illustrative embodiment, short-SF despreader 512 generates eachsingle interference measurement value using a fast Hadamard transform(FHT) over the unoccupied code symbol. In this way, noise estimator 516can generate a new noise estimate after every code symbol periodcorresponding to the unoccupied code.

As discussed above, where the subscriber station 102 knows that theC(128,1) code 206 is unoccupied, and where the P-CCPCH channel istransmitted in only nine out of ten 256-chip code symbol periods,control processor 520 reconfigures short-SF despreader 512 and noiseestimator 516 in order to perform optimal noise measurements for each256-chip code symbol period. Specifically, control processor 520configures short-SF despreader 512 and noise estimator 516 to generateone noise estimate every 128 chips during the 90% of the time that theP-CCPCH channel is being transmitted. Control processor 520 reconfiguresshort-SF despreader 512 and noise estimator 516 to generate one noiseestimate every 64 chips during the 10% of the time that the P-CCPCHchannel is not being transmitted.

Also discussed above is how the subscriber station can generatedimproved channel estimates based on knowledge that both the P-CCPCH 204and C(128,1) 206 codes are unoccupied, at least during one 256-chipperiod out of every ten. In an illustrative embodiment, the output ofdescrambler 508 is provided to a channel estimator 522. Channelestimator 522 generates one channel estimate over each code symbolperiod of the C(64,0) code 208. Each channel estimate includes bothamplitude and phase information, which are used to improve theperformance of coherent demodulation of the various downlink channelsignals. Channel estimator 522 can generate each of these channelestimates by filtering the output of descrambler 508, for example usingIIR or FIR filtering. In an illustrative embodiment, channel estimator522 further includes additional secondary filters for filtering the64-chip channel estimates. In an illustrative embodiment, thesesecondary filters may be IIR or FIR filters. One example of an FIRfilter is averaging every four 64-chip channel estimates in each256-chip code symbol period to form a combined channel estimate forevery 256-chip code symbol period.

The identification of unoccupied codes can be accomplished in any ofseveral ways. For example, the identity of one or more unoccupied codescan be transmitted within a signal on a control channel received by thesubscriber station 102. As discussed above, where a base station needsonly to indicate whether the C(128,1) code 206 is unoccupied, this canbe accomplished using a single information bit. For example, thetransmission of a ‘1’ could indicate that the C(128,1) code 206 isunoccupied, where transmission of a ‘0’ could indicate that the C(128,1)code is being used to transmit data. The single bit may be transmittedfrom the base station over a control channel and demodulated usingcontrol channel demodulator 510. In an illustrative embodiment, theoutput of descrambler 508 is provided to control channel demodulator510, which demodulates the single information bit to determine whetherits value is a ‘1’ or a ‘0’ and provides the decoded bits to controlprocessor 520. Control processor 520 adjusts the control signals toshort-SF despreader 512 and to noise estimator 516 based on the value ofthe information bit.

The control channel signal may also include data that identifies anunoccupied code that is outside the code tree branch C(64,0) 208.Control channel demodulator 510 demodulates the control channel signaland provides the data indicative of the unoccupied code to controlprocessor 520. Control processor 520 then adjusts the control signals toshort-SF despreader 512 and noise estimator 516 directing them togenerate noise estimates utilizing the unoccupied code. Noise estimator516 provides the noise estimates to control processor 520.

As discussed above, the control channel whose signals are demodulated bycontrol channel demodulator 510 may be a broadcast channel, a channeldedicated to the subscriber station 102, or a multicast channel used totransmit information to a plurality of subscriber stations that includessubscriber station 102. Where the subscriber station 102 is configuredto perform multiple types of noise estimation, such as the differenttypes of noise estimation shown in FIGS. 3 and 4, the different types ofunoccupied-channel information may be carried on channels of differenttypes. For example, the single bit indicating the usage of the C(128,1)code 206 may be transmitted using a broadcast channel, where anothermessage identifying a different unoccupied code might be carried on adedicated channel.

Many different formats for identifying an unoccupied code areanticipated herein. In an illustrative embodiment, a control channel isused to transmit the spreading factor and identity of an unused code.For example, a code C(X,Y) could be identified using a bit-mapped 8-bitvalue including two bits to identify X and six bits to identify Y. Usingthis kind of a code, the two bits would be sufficient to indicate any ofthe code lengths from 16 to 128 chips or from 8 to 64 chips. The sixbits could provide an index to any of 64 codes within the groupidentified by the first two bits. Alternatively, a set of codes could beidentified using fewer control channel bits to identify one of apredetermined set of short-SF codes that are designated as seldom-used.For example, the base station could include a rule that C(32,31) 218,C(64,63) 216, and C(128,127) 214 are always to be allocated last. Any ofthese codes could be indicated using a two-bit message. For example,‘00’ could indicate that none of the channels are unoccupied, ‘01’ couldindicate that C(32,31) is unoccupied and so on. As yet another possiblealternative, the usage of a single predetermined short-SF code, forexample the 64-chip code C(64,1) (not shown), could be indicated using asingle bit. Alternatively, the occupied or unoccupied status of anyother predetermined code could be indicated using a single bit assignedspecifically to that predetermined code.

Alternatively, the apparatus shown in FIG. 5 could also be used toidentify an unused code channel without receiving such information overa control channel. In an illustrative embodiment, the subscriber station102 utilizes the apparatus in FIG. 5 to perform an energy-thresholdsearch over some or all of the possible 64-chip codes.

FIG. 6 shows an illustrative base station apparatus for providingtransmission signals that allow noise estimation using short-SF codes.The unmodulated CPICH signal 602 is provided to a summer 618. TheP-CCPCH signal 604 is modulated using the P-CCPCH code 204 in a P-CCPCHchannel modulator 628. The modulated P-CCPCH signal is then provided toa gain controller 610 that individually adjusts the gain of themodulated P-CCPCH signal. In an illustrative embodiment, switch 612periodically gates the transmission of the P-CCPCH channel, for exampleat a 90% duty-cycle in which the channel is transmitted 90% of the timeand not transmitted the remaining 10% of the time. In an illustrativeembodiment, switch 612 opens for one out of every ten 256-chip codesymbol periods.

Data signals 606 are provided to one or more data channel modulators614, which modulate the individual data signals using code symbols ofvarying code symbol lengths and spreading factors to provide modulateddata signals. After modulation in modulators 614, each of the datasignals is individually gain-adjusted by a gain controller 610. At leastone unoccupied-channel signal provided by control processor 608 ismodulated using at least one control channel modulator 616 to provide amodulated control channel signal. The modulated control channel signalis then gain-adjusted by gain controller 610. In an illustrativeembodiment, control processor 608 provides gain control signals thatdetermine the gain to be applied to each signal by gain controllers 610.Alternatively, the gain control signals could be provided by anadditional control processor (not shown).

Control processor 608 provides the unoccupied-channel signal based onwhich codes in code tree 220 are occupied. The unoccupied-channel signalor signals can be formatted in any of the ways described above inconjunction with FIG. 5. For example, a single bit may be used toindicate whether code C(128,1) 206 is occupied. Alternatively, a singlebit may be used to indicate whether a single 64-chip code other thanC(64,0) 208, for example the 64-chip code C(64,1) (not shown), isoccupied. Or, multiple bits may be used to identify one of a set ofpossible unoccupied codes of varying spreading factors.

In an illustrative embodiment, control processor 608 allocates codechannels for use in transmitting data signals to subscriber stations. Inorder to maximize the probability that a short-SF code is available forsubscriber stations to use for noise measurements, control processor 608allocates predetermined short-SF codes designated as seldom-used onlyafter all other codes of the same SF are already occupied. For example,as described above, control processor 608 could keep C(32,31) 218,C(64,63) 216, and C(128,127) 214 unoccupied as often as possible andindicate the shortest-SF unoccupied code using a two-bit message. Forexample, ‘00’ could indicate that none of the channels are unoccupied,‘01’ could indicate that C(32,31) is unoccupied and so on. Controlprocessor 608 holds any designated seldom-used short-SF code unoccupiedas often as possible.

In an illustrative embodiment, the gain-adjusted CPICH, P-CCPCH,modulated data signals, and modulated control signals are added togetherin summer 618, which sums the signals into a combined transmit signal.The combined transmit signal is then multiplied by a scrambling code inscrambler 622 to provide a scrambled signal. Scrambling code generator620 provides the scrambling code to scrambler 622. The scrambling codeprovided by scrambling code generator 620 should be the same as thescrambling code generated by the scrambling code generator 518 in thesubscriber station 102. Accordingly, and as discussed above, thescrambling code provided by scrambling code generator 620 may be a realor complex code. Also, the scrambling code may be a pseudonoise (PN)code. Where the combined transmit signal provided from summer 618 is acomplex signal, scrambler 622 performs complex multiplication toscramble the combined transmit signal. The scrambled signal generated byscrambler 622 is upconverted in upconverter (UCVT) 624, amplified inhigh-power amplifier (HPA) 626, and transmitted through antenna 114. Asmentioned above, base station 112 may use multiple antennas withoutdeparting from the embodiments described herein.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality.Whether such functionality is implemented as hardware or softwaredepends upon the particular application and design constraints imposedon the overall system. Skilled artisans may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a digital signalprocessor (DSP), an application specific integrated circuit (ASIC), afield programmable gate array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.Such implementations may be applied, for example, to the control channeldemodulator 510, short-SF despreader 512, noise estimator 516, andcontrol processor 520. Furthermore, a general purpose processor may be amicroprocessor, but in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices,e.g., a combination of a DSP and a microprocessor, a plurality ofmicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processorcan read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a subscriber station. In the alternative, the processor andthe storage medium may reside as discrete components in a subscriberstation.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A noise measurement method comprising:determining that a short-spreading-factor code is unoccupied, whereinthe short-spreading-factor code has a short spreading factor that isless than a longest spreading factor of an occupiedlongest-spreading-factor code associated with a longest-spreading factortime period; measuring a first symbol energy, during a first short timeperiod, based on the short-spreading-factor code to provide a firstmeasured symbol energy; measuring a second symbol energy, during asecond short time period, based on the short-spreading-factor code toprovide a second measured symbol energy, wherein the first and secondshort time periods occupy different time periods within thelongest-spreading-factor time period, wherein the first symbol energycorresponds to an energy associated with a symbol having a first symboltype and the second symbol energy corresponds to an energy associatedwith a symbol having the first symbol type; and generating a first noiseestimate based on a difference between the first measured symbol energyand the second measured symbol energy.
 2. The method of claim 1 furthercomprising receiving a message indicating that theshort-spreading-factor code is unoccupied, wherein the determining isbased on the message.
 3. The method of claim 2 wherein the message is asingle bit, and wherein the short-spreading-factor code is a C(128,1)code, and wherein the short spreading factor is
 128. 4. The method ofclaim 2 wherein the message is a single bit, and wherein theshort-spreading-factor code has a length less than or equal to 128 chipsand is a predetermined code other than a C(128,1) code.
 5. The method ofclaim 2 wherein the message is a single bit, and wherein theshort-spreading-factor code is a predetermined code having a lengthequal to 64 chips.
 6. The method of claim 2 wherein the message is asingle bit, and wherein the short-spreading-factor code is apredetermined code having a length equal to 32 chips.
 7. The method ofclaim 2 wherein the message has more than one bit and identifies theshort-spreading-factor code from a set of codes having varying chiplengths.
 8. The method of claim 1 wherein the short-spreading-factorcode shares a common parent code with at least onelongest-spreading-factor code that is occupied, and wherein thelongest-spreading-factor code has a spreading factor equal to thelongest spreading factor.
 9. The method of claim 1 wherein the shortspreading factor is equal to one half of the longest spreading factor,and wherein the short-spreading-factor code shares a common parent codewith two longest-spreading-factor codes, wherein a first of the twolongest-spreading-factor codes is always occupied, and wherein thelongest-spreading-factor codes both have a spreading factor equal to thelongest spreading factor.
 10. The method of claim 1 wherein the firstshort time period occupies a first half of the longest-spreading-factortime period, and second short time period occupies a second half of thelongest-spreading-factor time period.
 11. The method of claim 1, whereinthe measured first symbol energy and the measured second symbol energyare each measured in response to determining that the short-spreadingfactor code is unoccupied.
 12. The method of claim 1, wherein the firstnoise estimate includes an estimate of a thermal noise associated with achannel over which a first received symbol corresponding to the firstsymbol energy is transmitted and over which a second received symbolcorresponding to the second symbol energy is transmitted.
 13. The methodof claim 1, wherein the first symbol energy corresponds to an energyassociated with an orthogonal code at a first time, and wherein thesecond symbol energy corresponds to an energy associated with theorthogonal code at a second time.
 14. The method of claim 1, wherein theshort-spreading-factor code corresponds to a first chip length, whereinthe longest-spreading factor code corresponds to a second chip length,and wherein the first chip length and the second chip length aredifferent.
 15. An apparatus comprising: means for determining that ashort-spreading-factor code is unoccupied, wherein theshort-spreading-factor code has a short spreading factor that is lessthan a longest spreading factor of an occupied longest-spreading-factorcode associated with a longest-spreading factor time period; means formeasuring a first symbol energy, during a first short time period, basedon the short-spreading-factor code to provide a first measured symbolenergy; means for measuring a second symbol energy, during a secondshort time period, based on the short-spreading-factor code to provide asecond measured symbol energy, wherein the first and second short timeperiods occupy different time periods within thelongest-spreading-factor time period, wherein the first symbol energycorresponds to an energy associated with a symbol having a first symboltype and the second symbol energy corresponds to an energy associatedwith a symbol having the first symbol type; and means for generating afirst noise estimate based on a difference between the first measuredsymbol energy and the second measured symbol energy.
 16. An apparatuscomprising: a control processor configured to determine that ashort-spreading-factor code is unoccupied, wherein theshort-spreading-factor code has a short spreading factor that is lessthan a longest spreading factor of an occupied longest-spreading-factorcode associated with a longest-spreading factor time period; adespreader configured to measure a first symbol energy, during a firstshort time period, based on the short-spreading-factor code to provide afirst measured symbol energy, and to measure a second symbol energy,during a second short time period, based on the short-spreading-factorcode to provide a second measured symbol energy, wherein the first andsecond short time periods occupy different time periods within thelongest-spreading-factor time period, wherein the first symbol energycorresponds to an energy associated with a symbol having a first symboltype and the second symbol energy corresponds to an energy associatedwith a symbol having the first symbol type; and a noise estimatorconfigured to generate a first noise estimate based on a differencebetween the first measured symbol energy and the second measured symbolenergy.
 17. The apparatus of claim 16 further comprising a controlchannel demodulator configured to demodulate a message indicating thatthe short-spreading-factor code is unoccupied, wherein the controlprocessor determines that the short-spreading-factor code is unoccupiedbased on the message.
 18. The apparatus of claim 17 wherein the controlchannel demodulator is configured to decode the message as a single bit,and wherein the despreader is configured to measure the first and secondsymbol energy using a C(128,1) code for the short-spreading-factor. 19.The apparatus of claim 17 wherein the control channel demodulator isconfigured to decode the message as a single bit, and wherein thedespreader is configured to measure the first and second symbol energyusing a predetermined short-spreading-factor code associated with thesingle bit, wherein the predetermined short-spreading-factor code has alength less than or equal to 128 chips.
 20. The apparatus of claim 17wherein the control channel demodulator is configured to decode themessage as a single bit, and wherein the despreader is configured tomeasure the first and second symbol energy using a predeterminedshort-spreading-factor code associated with the single bit, wherein thepredetermined short-spreading-factor code has a length equal to 64chips.
 21. The apparatus of claim 17 wherein the control channeldemodulator is configured to decode the message as a single bit, andwherein the despreader is configured to measure the first and secondsymbol energy using a predetermined short-spreading-factor codeassociated with the single bit, wherein the predeterminedshort-spreading-factor code has a length equal to 32 chips.
 22. Theapparatus of claim 17 wherein the message that the control channeldemodulator is configured to decode has more than one bit, and whereinthe despreader is configured to measure the first and second symbolenergy using any of a predetermined set of codes having varying chiplengths based on the more than one bit.
 23. The apparatus of claim 17,wherein the control channel demodulator is further configured to decodethe message as a single bit.
 24. The apparatus of claim 17, wherein thedespreader is further configured to measure the first symbol energy andthe second symbol energy using a C(128,1) code for theshort-spreading-factor.
 25. The apparatus of claim 17, wherein theshort-spreading-factor code has a length that is less than or equal to128 chips, equal to 64 chips, or equal to 32 chips.
 26. The apparatus ofclaim 16 wherein the first short time period occupies a first half ofthe longest-spreading-factor time period, and second short time periodoccupies a second half of the longest-spreading-factor time period. 27.A computer-readable medium embodying instructions which, when executedby a processor, cause said processor to perform a noise measurementmethod comprising: determining that a short-spreading-factor code isunoccupied, wherein the short-spreading-factor code has a shortspreading factor that is less than a longest spreading factor of anoccupied longest-spreading-factor code associated with alongest-spreading factor time period; providing a control signal to adespreader to cause the despreader to measure a first symbol energy,during a first short time period, based on the short-spreading-factorcode to provide a first measured symbol energy, and to measure a secondsymbol energy, during a second short time period, based on theshort-spreading-factor code to provide a second measured symbol energy,wherein the first symbol energy corresponds to an energy associated witha symbol having a first symbol type and the second symbol energycorresponds to an energy associated with a symbol having the firstsymbol type; and providing a control signal to a noise estimator tocause the noise estimator to generate a first noise estimate based on adifference between the first measured symbol energy and the secondmeasured symbol energy.
 28. A noise measurement method, comprising:determining that a short-spreading-factor code is unoccupied, whereinthe short-spreading-factor code has a short spreading factor that isless than a longest spreading factor; measuring a first symbol energybased on a parent code of the short-spreading-factor code to provide afirst measured symbol energy, the first symbol energy being measured ina first parent code symbol period; measuring a second symbol energybased on the parent code of the short-spreading-factor code to provide asecond measured symbol energy, the second symbol energy being measuredin a second parent code symbol period, wherein data in any occupiedchild codes of the parent code of the short-spreading-factor code is thesame in the first parent code symbol period and the second parent codesymbol period; and generating a first noise estimate based on adifference between the first measured symbol energy and the secondmeasured symbol energy.
 29. The method of claim 28, further comprisingreceiving a message indicating that the short-spreading-factor code isunoccupied, wherein determining that the short-spreading-factor code isunoccupied is based on the message.
 30. The method of claim 29, whereinthe message is a single bit, wherein the short-spreading-factor code isa C(128,1) code, and wherein the short spreading factor is
 128. 31. Themethod of claim 29, wherein the message is a single bit, and wherein theshort-spreading-factor code has a length that is less than or equal to128 chips and is a predetermined code other than a C(128,1) code. 32.The method of claim 29, wherein the message is a single bit, and whereinthe short-spreading-factor code is a predetermined code having a lengthequal to 64 chips.
 33. The method of claim 29, wherein the message is asingle bit, and wherein the short-spreading-factor code is apredetermined code having a length equal to 32 chips.
 34. The method ofclaim 29, wherein the message has more than one bit and identifies theshort-spreading-factor code from a set of codes having varying chiplengths.
 35. The method of claim 28, wherein the short-spreading-factorcode shares a common parent code with at least onelongest-spreading-factor code that is occupied, and wherein thelongest-spreading-factor code has a spreading factor equal to thelongest spreading factor.
 36. The method of claim 28, wherein the shortspreading factor is equal to one half of the longest spreading factor,and wherein the short-spreading-factor code shares a common parent codewith two longest-spreading-factor codes, wherein a first of the twolongest-spreading-factor codes is always occupied, and wherein thelongest-spreading-factor codes both have a spreading factor equal to thelongest spreading factor.
 37. The method of claim 36, furthercomprising: determining that a second of the twolongest-spreading-factor codes is unoccupied at a predeterminedduty-cycle; measuring a third symbol energy based on the parent code ofthe short-spreading-factor code to provide a third measured symbolenergy; measuring a fourth symbol energy based on the parent code of theshort-spreading-factor code to provide a fourth measured symbol energy;and generating a second noise estimate based on a difference between thethird measured symbol energy and the fourth measured symbol energy,wherein the measuring a first symbol energy, the measuring a secondsymbol energy, the measuring a third symbol energy, and the measuring afourth symbol energy occur respectively in four consecutive code symbolperiods existing within a longest code symbol period associated with alongest code symbol having the longest spreading factor.
 38. Acomputer-readable medium embodying instructions which, when executed bya processor, cause said processor to: determine that ashort-spreading-factor code is unoccupied, wherein theshort-spreading-factor code has a short spreading factor that is lessthan a longest spreading factor; measure a first symbol energy based onthe parent code of the short-spreading-factor code to provide a firstmeasured symbol energy, the first symbol energy being measured in afirst parent code symbol period; measure a second symbol energy based ona parent code of the short-spreading-factor code to provide a secondmeasured symbol energy, the second symbol energy being measured in asecond parent code symbol period, wherein data in any occupied childcodes of the parent code of the short-spreading-factor code is the samein the first parent code symbol period and the second parent code symbolperiod; and generate a first noise estimate based on a differencebetween the first measured symbol energy and the second measured symbolenergy.
 39. An apparatus, comprising: means for determining that ashort-spreading-factor code is unoccupied, wherein theshort-spreading-factor code has a short spreading factor that is lessthan a longest spreading factor; means for measuring a first symbolenergy based on a parent code of the short-spreading factor code toprovide a first measured symbol energy, the first symbol energy beingmeasured in a first parent code symbol period; means for measuring asecond symbol energy based on the parent code of the shortspreading-factor code to provide a second measured symbol energy, thesecond symbol energy being measured in a second parent code symbolperiod, wherein data in any occupied child codes of the parent code ofthe short-spreading-factor code is the same in the first parent codesymbol period and the second parent code symbol period; and means forgenerating a first noise estimate based on a difference between thefirst measured symbol energy and the second measured symbol energy. 40.The apparatus of claim 39, wherein the means for determining comprises acontrol processor, and wherein the means for measuring comprises adespreader configured to measure the first symbol energy and the secondsymbol energy.
 41. The apparatus of claim 40, further comprising acontrol channel demodulator configured to demodulate a messageindicating that the short-spreading-factor code is unoccupied, whereinthe control processor determines that the short-spreading-factor code isunoccupied based on the message.
 42. The apparatus of claim 41, whereinthe control channel demodulator is configured to decode the message as asingle bit, and wherein the despreader is configured to measure thefirst and second symbol energy using a C(128,1) code for theshort-spreading-factor.
 43. The apparatus of claim 41, wherein thecontrol channel demodulator is configured to decode the message as asingle bit, wherein the despreader is configured to measure the firstand second symbol energy using a predetermined short-spreading-factorcode associated with the single bit, and wherein the predeterminedshort-spreading-factor code has a length less than or equal to 128chips.
 44. The apparatus of claim 41, wherein the control channeldemodulator is configured to decode the message as a single bit, whereinthe despreader is configured to measure the first and second symbolenergy using a predetermined short-spreading-factor code associated withthe single bit, and wherein the predetermined short-spreading-factorcode has a length equal to 64 chips.
 45. The apparatus of claim 41,wherein the control channel demodulator is configured to decode themessage as a single bit, wherein the despreader is configured to measurethe first and second symbol energy using a predeterminedshort-spreading-factor code associated with the single bit, and whereinthe predetermined short-spreading-factor code has a length equal to 32chips.
 46. The apparatus of claim 41, wherein the message that thecontrol channel demodulator is configured to decode has more than onebit, and wherein the despreader is configured to measure the first andsecond symbol energy using any of a predetermined set of codes havingvarying chip lengths based on the more than one bit.
 47. The apparatusof claim 40, wherein the short-spreading-factor code has a spreadingfactor that is equal to one quarter of the longest spreading factor, andwherein the despreader is further configured to measure four symbolenergies over four consecutive short-spreading-factor code symbolperiods within a long-spreading-factor-code symbol period to provide athird measured symbol energy and a fourth measured symbol energy inaddition to the first and second symbol energy.
 48. The apparatus ofclaim 47, wherein the noise estimator is further configured to providetwo noise estimates for every long-spreading-factor-code symbol period,the first of the two noise estimates comprising the first noise estimateand a second noise estimate, by determining a difference between thethird measured symbol energy and the fourth measured symbol energy. 49.The apparatus of claim 48, wherein the control processor is furtherconfigured to determine when, during a predetermined duty-cycle, alongest-spreading-factor code is unoccupied.